CANCER-RELATED BEHAVIOR OF VITAMIN SUPPLEMENT USERS

Epidemiological studies suggest that certain vitamin supplements may r educe the risk of some cancers. However, observational studies can be compromised by confounding, because supplement use is related to other factors that affect cancer risk. The purpose of this paper is to iden tify cancer-related behaviors that could confound studies of the assoc iations between vitamin supplement use and cancer risk. Data are from a random digit dial survey to monitor cancer risk behavior in adults i n Washington State (n = 1449). Unconditional logistic regression was u sed to examine whether regular supplement users were more likely to pr actice other cancer-related behaviors than nonusers, after adjustment for age, education, and smoking. Among women, supplement users were mo re likely to have had a sigmoidoscopy [odds ratio (OR), 2.3; 95% confi dence interval (CI), 1.2-4.5], hemoccult (OR, 2.3; CI, 1.5-3.5), or ma mmogram (OR, 1.5; CI, 1.0-2.1) in the past 2 years. Among men, supplem ent users were twice as likely to have had a prostate-specific antigen test (OR, 2.2; CI, 1.3-3.7) and to regularly take aspirin (OR, 1.7; C I, 1.1-2.6). Supplement users were statistically significantly more li kely to exercise regularly, eat four or more servings of fruits and ve getables per day, follow a low-fat diet pattern, and believe in a conn ection between diet and cancer. The association was especially strong for fruits and vegetables (women, OR, 1.9; and CI, 1.3-2.6; men, OR, 2 .4; CI, 1.6-3.8). Those investigating the benefits and risks of vitami n and mineral supplements need to be aware of the lifestyle characteri stics of supplement users to assess the potential for bias in their st udies.